Arlington’s Storm Water Budget Balloons Due to Runoff Inducing Tree Removal

Comments at Capital Budget (CIP) Hearing, June 30, 2020.

The proposed FY 2021 CIP shows a refreshing change of direction from funding overbuilt boondoggles to paying down basic infrastructure. The County Manager estimates that up to $200 million will be spent on stormwater facilities over the next ten years (p. 11).

No one who witnessed the July 8, 2019 flood would doubt the wisdom of this critical investment. What I quarrel with is the cost. Much of the infrastructure to be constructed consists of low-impact-development (LID) Best Management Practice (BMP) facilities such as: pavers, cisterns, bioretention basins, swales, planters, green roofs and rain gardens.

According to Stormwater Magazine (Table 1), the average capital cost of LID BMPs was around $6.40 per gallon in 2013 or about $7 today.

According to the National Tree Benefits Calculator the average 40 inch hardwood will intercept 19,000 gallons of storm water runoff per year. Assume for the sake of argument that a LID facility needs to capture only ten percent of that amount in one major rain event each year.

Based on the following additional assumptions:

  • 1000 trees recently removed from public property
  • O&M costs at 10% of capital cost
  • system maintenance for ten years

a lowball estimate of the cost of the additional stormwater facilities needed to mitigate runoff due to tree removal is:

  • $13.3 million in capital costs
  • $13.3 million O&M for 10 years of operations, or $26.6 million total.

This may not sound like a lot in a ten-year CIP pegged at $330 million (Appendix A). But it is money that could be better spent on schools, parks, street paving, and walkable streets.[i]

[i] Capital Cost=19,000 gal. runoff/yr. * $7/gal. * .1 capture factor * 1,000 mature trees=$13.3 million

Operating Cost=$13.3 million capital cost *.1 O&M factor * 10 years=$13.3 million